As an information display device substitutable for liquid crystal display (LCD), information display devices with the use of technology such as an electrophoresis method, an electro-chromic method, a thermal method, dichroic-particles-rotary method have been proposed.
These conventional techniques are considered to be useful for inexpensive visual display devices of the next generation compared with a LCD due to merits such as wider field of vision close to normal printed matter, smaller power consumption or a memory function, and expected to be spread out to information displays for portable devices, electronic paper and the like. Recently, electrophoresis method for microencapsulating dispersion liquid made up with dispersion particles and coloration solution and disposing the liquid between opposed substrates, is proposed and expected.
However, in the electrophoresis method, there is a problem that a response rate is slow by the reason of viscosity resistance because the particles migrate among the electrophoresis solution. Further, there is a problem of lacking imaging repetition stability, because particles with high specific gravity of titanium oxide is scattered within solution of low specific gravity and it is difficult to maintain a stability of dispersion state. Even in the case of microencapsulating, the cell size is diminished to a microcapsule level in order to make it hard to show the above-mentioned drawbacks, however, an essential problem is not overcome at all.
Besides the electrophoresis method using behavior in the solution, recently, a method that electro-conductive particles and a charge transport layer are installed in a part of the substrate without using solution has been proposed. [The Imaging Society of Japan “Japan Hardcopy '99” (Jul. 21-23, 1999) Transaction Pages 249-252] However, the structure becomes complicated because the charge transport layer and further a charge generation layer are to be arranged. In addition, it is difficult to constantly charge the electro-conductive particles, and thus there is a drawback on the lack of stability.
As one method for overcoming the various problems mentioned above, an information display device is known, wherein the display media are sealed between two opposed substrates, at least one of which is transparent, and wherein the display media, to which an electrostatic field is applied, are made to move so as to display information such as an image or the like.
The particles used for the above information display device need to have fluidity and controllability of charge capacity. Apart from the present invention, there is a known technique that the fluidity is improved and the charge capacity can be controlled by means of attaching the child (minor) particles (extra additive agents such as titania or fumed silica fine particles) to the mother (major) particle. However, when the electrophotographic toner such as the above-mentioned toner extra additive agent attaching type is used as the display media for the above-mentioned information display device, the child particles (extra additive agents) are easily dissociated from the surface of the mother particle, or the child particles are completely embedded into the mother particle due to thermal and mechanical stress arising in a reverse durability test of repeating the reverse of the information display. Thus, there was a problem that the primary performance cannot be maintained during a reverse durability test.